FIG. 1 depicts a schematic diagram of a wireless local-area network 100 in the prior art, which comprises: access point 101, stations 102-1 through 102-N, wherein N is a positive integer, and hosts 103-1 through 103-N, interconnected as shown. Each station 102-i, wherein i is a member of the set {1, 2, . . . N}, enables host 103-i (a device such as a notebook computer, personal digital assistant [PDA], tablet PC, etc.) to communicate wirelessly with other hosts in local-area network 100 via access point 101.
Stations 101-1 through 101-N and access point 102 transmit blocks of data, called frames, to each other, via a shared-communications channel such that if two or more stations (or an access point and a station) transmit frames simultaneously via the shared-communications channel, then one or more of the frames can become corrupted (resulting in a collision). Consequently, local-area networks typically employ protocols for ensuring that a station can gain exclusive access to the shared-communications channel for an interval of time in order to transmit frames.
The Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of protocols for wireless local-area networks employs a variety of mechanisms for avoiding and recovering from such collisions. One such mechanism, the Distributed Coordination Function (DCF), is a contention-based access mechanism in which stations, including the access point, compete to gain exclusive access to the shared-communications channel, just as, for example, several children might fight to grab a telephone to make a call.
In accordance with the Distributed Coordination Function, each station with frames to transmit (including the access point) waits until the shared-communications channel has been idle for a pre-determined time interval before transmitting into the channel. (Stations detect whether the shared-communications channel is idle or busy via a carrier-sensing mechanism.) In IEEE 802.11e, a variation of 802.11 that provides quality-of-service (QoS), frames are assigned to different traffic categories, and the length of time that a station waits before transmitting a frame into the channel, called the Arbitration Interframe Space (AIFS), depends on the traffic category to which the frame belongs.
After a station, including the access point, determines that the shared-communications channel has been sufficiently idle, the station selects a random time slot in which to transmit frames from a back-off contention window (sometimes referred to simply as a back-off window or a contention window). If two or more stations select the same time slot in the back-off contention window, then a collision occurs. When a transmitting station detects the collision, the station repeats the back-off procedure with a new, larger contention window (limited to a maximum size) and re-transmits the frames accordingly. By expanding the contention window after a collision, the probability that two or more stations will select the same time slot, and therefore experience another collision, is reduced.